Polyester film, laminated metal sheet and metal container

ABSTRACT

A polyester film having a coefficient of dynamic friction between the film and a metal at 80° C. of not more than 0.45, a polyester film composed of a polyester comprising an ethylene terephthalate unit in a proportion of 70% or more by mole, the polyester film comprising a cyclic ethylene terephthalate trimer in a proportion of 0.7% or less by weight, laminated metal sheets and metal containers manufactured from said laminated metal sheet. According to the present invention, the polyester film shows superior sliding property and superior scratch resistance during can production, as well as the elution of oligomer from the film can be suppressed, so that when the film is used for metal containers for food, the change in taste and smell of the food contained therein and degraded appearance of the surface of the film caused by the deposition of oligomer are avoided.

FIELD OF THE INVENTION

The present invention relates to a polyester film for laminating with ametal material mainly usable for food such as soft drinks, beer andcanned food, a metal sheet laminated with said film and to metalcontainers produced by forming said laminated metal sheet into a canshape. More particularly, the present invention relates to a polyesterfilm with superior scratch resistance during can production process toafford good productivity of the cans, a metal sheet laminated with saidfilm and to metal containers produced by forming said laminated metalsheet into a can shape. Moreover, the present invention relates to apolyester film with suppressed elution of low molecular substances fromthe film due to a heat treatment after packing the food therein, such asretort treatment, a metal sheet laminated with said film and to metalcontainers produced by forming said laminated metal sheet into a canshape.

BACKGROUND OF THE INVENTION

For inhibiting corrosion of the inner and outer surfaces of metal cans,paints are conventionally applied, wherein used as the paints arethermosetting resins.

Other methods include the use of thermoplastic resins. For example, apolyolefin film is laminated with a tin-free steel heated or a polyesterfilm with good heat resistance is laminated with a metal sheet to beused for cans.

Many thermosetting paints are of solvent type. For forming a coatingtherefrom, a high temperature heating for a long time at 150°-250° C.for several minutes and baking which undesirably causes evaporation of alarge amount of organic solvent are necessary. Accordingly,simplification of steps and improvements to prevent pollution aredesired. It is inevitable, moreover, that a small amount of organicsolvent remains in the coating when the thermosetting paints are appliedunder the above-mentioned conditions and the remainder of the organicsolvent migrates into the food packed in a metal can applied with thecoating to result in degraded taste and smell of the food. Also, theadditives in the paint or low molecular substances which may be producedby an incomplete crosslinking reaction may move into the food to exertthe same adverse influences.

Of the above-mentioned problems, simplification of the steps andprevention of pollution can be achieved by the use of thermoplasticresin films. However, polyolefin films such as those made frompolyethylene and polypropylene, which are among thermoplastic resins,have poor heat resistance and sometimes whiten and peel off from a metalsheet upon a retort treatment. In addition, polyolefin films are softand poor in scratch resistance. The poor scratch resistance of the filmcauses problems that, for example, scratches occur on the film surfaceduring transporting each laminated metal sheet or seaming sheets into acan in a can manufacture process, thus impairing the product value.

While the problem caused by the migration of residual solvent asobserved when a thermosetting paint is applied can be overcome by theuse of polyolefin films, the migration of low molecular substancesproduced during forming a coating or that of additives such as a heatstabilizer into food still exerts disadvantageous effects on taste andsmell of the food. The polyolefin film, moreover, adsorbs aromaticingredients in the food and is poor in flavor resistance.

On the other hand, the use of a polyester film from among thethermoplastic resin films is most preferable because the above-mentioneddifficulties which polyolefin films encounter can be overcome.

Although polyester films generally show superior scratch resistance ascompared with polyolefin films, the resistance is not entirelysatisfactory and an improvement thereof is desired. So as to improve thescratch resistance, the application of an organic coating superior inlubricating property and scratch resistance to the surface of apolyester film has been proposed. While the scratch resistance can beunquestionably improved by this coating method, this method necessitatesthe use of an organic solvent during the process of forming an organiccoating and a very small portion of the solvent remains in the organiccoating obtained. Accordingly, when a food is packed in a metalcontainer manufactured from this film, the organic solvent adverselyaffects the taste and smell of the food. There is also a problem thatlow molecular substances elute out from the organic coating to exertadverse influence as does the residual organic solvent as described.

The polyester film is superior in heat resistance so that an additivesuch as a heat stabilizer is not necessary and it produces less amountsof low molecular substances. As a result, the problems of degraded tasteand smell of food caused by the migration of additives or low molecularsubstances can be markedly alleviated as compared with polyolefin films.The polyester films mainly comprising polyethylene terephthalate,however, contains low molecular substances produced duringpolymerization process and film forming process, i.e. oligomer basedmainly on a cyclic ethylene terephthalate trimer (hereinafter alsoreferred to merely as oligomer), and said oligomer may elute out fromthe film into food or may precipitate on the surface of a laminate film,giving rise to an appearance problem.

SUMMARY OF THE INVENTION

Accordingly, an object of the present invention is to provide apolyester film superior in scratch resistance, and a laminated metalsheet and a metal container produced by the use of said polyester film.

Another object of the present invention is to provide a polyester filmwith suppressed elution of oligomer, and a laminated metal sheet and ametal container produced by the use of said polyester film.

DETAILED DESCRIPTION OF THE INVENTION

Firstly, the present invention provides a polyester film having acoefficient of dynamic friction between the film and a metal at 80° C ofnot more than 0.45. A coefficient of dynamic friction between the filmand a metal at 80° C. of not more than 0.45 can contribute to thedesired superior scratch resistance. Particularly, the present inventionrelates to a polyester film composed of a polyester comprising anethylene terephthalate unit in a proportion of 70% or more by mole, thepolyester film comprising a cyclic ethylene terephthalate trimer in aproportion of 0.7% or less by weight; a polyester film made from apolyester composition comprising polymer particles of 0.5-5 μm inaverage particle size in a proportion of 0.3-5% by weight; a polyesterfilm made from a polyester composition comprising inorganic fineparticles of 0.5-5 μm in average particle size in a proportion of 0.3-5%by weight; and a polyester film made from a polyester compositioncomprising a thermoplastic resin incompatible with polyester in aproportion of 0.3-5% by weight. In addition, the present inventionrelates to a polyester film comprising a polyester layer (layer A)having a coefficient of dynamic friction between the film and a metal at80° C. of not more than 0.45 and a polyester layer (layer B) having amelting point of 180°-240° C.

Secondly, the present invention provides a polyester film composed of apolyester comprising an ethylene terephthalate unit in a proportion ofnot less than 70% by mole, wherein a cyclic ethylene terephthalatetrimer is contained in a proportion of not more than 0.7% by weight. Acyclic ethylene terephthalate trimer content of not more than 0.7% byweight can suppress the elution of an oligomer, achieving the secondobject of the invention.

Also, the present invention provides a polyester film composed of apolyester comprising a cyclic ethylene terephthalate trimer in aproportion of not more than 0.5% by weight, which is obtainable by solidpolymerization after heat-treating a polyester with a dihydric alcohol.

Further, the present invention provides a metal sheet laminated with thepolyester film of the above and metal containers manufactured from saidlaminated metal sheet.

The polyester to be used in the present invention is obtained bycondensation polymerization of a polycarboxylic acid and a polyhydricalcohol.

The polycarboxylic acid component includes, for example, dicarboxylicacid such as aromatic dicarboxylic acid (e.g. terephthalic acid,isophthalic acid, phthalic acid, naphthalene dicarboxylic acid, diphenyldicarboxylic acid), aliphatic dicarboxylic acid (e.g. adipic acid,azelaic acid, sebacic acid, decanedicarboxylic acid,dodecanedicarboxylic acid, dimer acid) and alicyclic dicarboxylic acid(e.g. cyclohexane-dicarboxylic acid). Of these, aromatic dicarboxylicacids such as terephthalic acid, isophthalic acid andnaphthalene-dicarboxylic acid are preferable in terms of flavorresistance.

The polyhydric alcohol component includes glycols such as aliphaticdiols (e.g. ethylene glycol, diethylene glycol, triethylene glycol,propanediol, butanediol, hexanediol, dodecanemethylene glycol, neopentylglycol), alicyclic diols (e.g. cyclohexanedimethanol) and aromatic diols(e.g. addition product of bisphenol derivative with ethylene oxide),with preference given to ethylene glycol.

It is preferable that 70% by mole or more and more preferably 80% bymole or more of the constituent components of said polyester should bean ethylene terephthalate unit. When the ethylene terephthalate unit isless than 70% by mole, heat resistance tends to become poor to oftencause stretching of a film when laminating with a metal to form a can,or reduction of width and occurrence of wrinkles due to the contractionby heat. As a result, laminating conditions may need to be modified soas to avoid such problems and productivity may become poor. In addition,the cost of the starting material polyester may become unbeneficiallyhigh.

As said polyester, polyethylene terephthalate or polyethyleneterephthalate added with 0.6-6% by weight (converted to polyethercomponent) of a polyester-polyol block copolymer is particularlypreferable. The polyester preferably suppresses whitening of film whichis caused by a treatment with hot water, such as a retort treatment.

From the aspect of dynamic properties, the polyester; preferably has anintrinsic viscosity of not less than 0.5, more preferably 0.55-0.85.

There is no limitation imposed on the production of the above-mentionedpolyester and the polyester can be produced by conventionally-knownester exchange or direct polymerization. The solid phase polymerizationmay be used for increasing the molecular weight. The solid phasepolymerization is preferable in that the content of the cyclic ethyleneterephthalate trimer to be mentioned later can be reduced.

The polyester film of the present invention preferably has a coefficientof dynamic friction between the film and a metal at 80° C. of not morethan 0.45, more preferably 0.40-0.20 and most preferably 0.35-0.20. Whenthe coefficient of dynamic friction is not more than 0.45, scratchresistance reaches a practical level and metal containers with goodappearance can be obtained even if the can manufacturing speed isincreased, thus enhancing the productivity of can manufacture.

So as to make the coefficient of dynamic friction not more than 0.45, apolyester composition comprising at least one component selected frompolymer particles of 0.5-5 μm in average particle size, inorganic fineparticles of 0.5-5 μm in average particle size and thermoplastic resinsincompatible with polyester, in a proportion of 0.3-5% by weight intotal may be used. The above-mentioned components may be used alone orin combination, with preference given to a combined use.

As the polymer particle, any suffices for use insofar as it stands thetemperature of melt-forming the polyester. For example, the polymerparticles may be obtained by addition polymerization, condensationpolymerization or polyaddition reaction. Examples of those obtained byaddition polymerization include polymers of acrylic monomers such asacrylic acid, methacrylic acid, acrylate and methacrylate; polymers ofstyrene monomers such as styrene and alkyl-substituted styrene;copolymers of acrylic monomer and styrene monomer; copolymers of acrylicmonomer and crosslinking monomer such as divinylbenzene, divinylsulfone,ethylene glycol dimethacrylate, trimethylolpropane trimethylacrylate andpentaerythritol tetramethylacrylate; copolymers of styrene monomer andcrosslinking monomer; and copolymers of acrylic monomer, styrene monomerand crosslinking monomer. Examples of those obtained by condensationpolymerization or polyaddition reaction include polymer particles ofmelamine resins, benzoguanamine resins, phenol resins and siliconeresins. Of these, preferred are polymer particles obtained fromcopolymers of acrylic monomer and/or styrene monomer and crosslinkingmonomer. Said polymer particles may be used alone or in combination.

The method for producing said polymer particles is not subject to anyparticular limitation and conventional emulsion polymerization andsuspension polymerization may be used. Pulverization or classificationmay be done on demand to adjust the particle size and particledistribution of said polymer particles.

The inorganic fine particles may be any insofar as they are insoluble inpolyester and are inert. Specific examples include metal oxides such assilica, alumina, zirconia and titanium oxide, compound oxides such askaolin, zeolite, sericite and sepiolite, sulfates such as calciumsulfate and barium sulfate, phosphates such as calcium phosphate andzirconium phosphate, and carbonates such as calcium carbonate. Theseinorganic fine particles may be natural or synthetic and are subject tono particular limitation on particle shape. The inorganic fine particlesmay be used alone or in combination. When the inorganic fine particlesalone are used, a combination of cohesive amorphous silica or sphericalsilica and zeolite is preferable.

Examples of thermoplastic resin incompatible with polyester includepolyolefin resins, polystyrene resins, polyacrylic resins, polycarbonateresins, polyamide resins, polysulfone resins and aromatic polyesterresins. The thermoplastic resin may be used alone or in combination. Inaddition, these resins need not be particles.

When the polymer particles and inorganic fine particles are used, anaverage particle size is preferably 0.5-5 μm, more preferably 0.8-4 μm.When it is less than 0.5 μm, sliding property between a film and a metalat high temperatures is not sufficiently improved and improvement inresistance to scratch tends to be unclear. An average particle size ofmore than 5 μm may pose saturation on the improvement in slidingproperty between a film and a metal at high temperatures, may causefalling off of fine particles on friction, and may result in rupturewhen forming a film.

The polymer particles, inorganic fine particles and thermoplastic resinincompatible with polyester are preferably contained in a proportion of0.3-5% by weight in total and more preferably 0.5-3% by weight in totalper the entire amount of a polyester composition. When they arecontained in a proportion of less than 0.3% by weight, improvement insliding property between a film and a metal at high temperatures becomessmall and scratch resistance tends to hardly manifest itself. Thecontent of more than 5% by weight, on the other hand, may posesaturation on the improvement in sliding property between a film and ametal at high temperature or may cause poor film formability.

The above-mentioned polymer particles, inorganic fine particles andthermoplastic resin incompatible with polyester may be added to apolyester during the manufacturing process of the polyester, or apolyester and the above-mentioned component may be mixed andmelt-kneaded. Alternatively, the above ingredients may be added in amaster batch containing them at high concentrations.

The polyester film of the present invention contains a cyclic ethyleneterephthalate trimer produced during polymerizing or forming a coating,preferably in a proportion of not more than 0.7% by weight per the totalamount of the polyester, by which the elution of an oligomer can befurther suppressed. More preferably, it is contained in a proportion ofnot more than 6% by weight and most preferably in a proportion of notmore than 5% by weight. When the cyclic ethylene terephthalate trimer iscontained in a proportion of more than 0.7% by weight, elution of anoligomer from a film tends to increase due to the heat treatment afterpacking the food (retort treatment). When the inside of a can islaminated with the film, the oligomer migrates into the food and tendsto exert an adverse influence on the taste and smell of the food. On theother hand, when the outside of a can is laminated with the film, theoligomer may deposit on the surface of the film to lose good appearanceof the can.

No particular limitation is imposed on the method for adjusting thecyclic ethylene terephthalate trimer content to not more than 0.7% byweight and the cyclic ethylene terephthalate trimer may be extracted andremoved from the film by the use of water or an organic solvent.Alternatively, a polyester containing a cyclic ethylene terephthalatetrimer in a less amount may be used as a starting material. The latteris more economical and preferable.

There is no particular limitation imposed on the method for producing apolyester with a smaller cyclic ethylene terephthalate trimer content,and heating under reduced pressure, solid phase polymerization,extraction with water or an organic solvent and a combination of thesemethods may be used. In particular, a method including the use of apolyester comprising a cyclic ethylene terephthalate trimer in aproportion of not more than 0.5% by weight, which is obtainable by solidpolymerization to be done after heat-treating a starting materialpolyester with a dihydric alcohol such as ethylene glycol, is mostdesirable. Different from conventional solid polymerization, this methodpermits reduction of the cyclic ethylene terephthalate trimer contentwithout increasing the polymerization degree of the polyester. The lowpolymerization degree which cannot be achieved by low oligomerpolyesters obtained by the conventional solid phase polymerizationresults in good melt flow of a polyester during the film formingprocess, reduced load on melt extruder and less facility cost for meltextrusion, thus providing a preferable mode of the embodiment.

The polyester film of the present invention may contain heat stabilizer,antioxidant, ultraviolet absorber, plasticizer, pigment, antistaticagent, lubricant, crystalline nucleus and so on demand.

The polyester film of the present invention may be either an unorientedfilm or an oriented film insofar as the aforementioned requirements aremet. In the case of an oriented film, it may be uniaxially oriented orbiaxially oriented, with preference given to a biaxially oriented filmin terms of isotropy.

The method for producing the polyester film is not limited. For example,an oriented film may be prepared by a conventional method such as aT-die method or a tubular method.

The polyester film may be mono-layered or plurally-layered. Anyplurally-layered film will suffice for use as long as the requirementsthat at least either outermost layer (at least the opposite side of thelayer in contact with the metal surface when forming a laminated metalsheet) shows a coefficient of dynamic friction between a film and ametal at 80° C. of not more than 0.45, and that the content of a cyclicethylene terephthalate trimer is not more than 0.7% by weight aresatisfied.

The polyester film preferably has a thickness of 3-100 μm, morepreferably 4-50 μm and most preferably 5-30 μm in the case of amonolayer film.

The polyester film of the present invention may be a composite film of apolyester layer (layer A) having a coefficient of dynamic frictionbetween a film and a metal at 80° C. of not more than 0.45 and apolyester layer (layer B) having a melting point of 180°-240° C.

The coefficient of dynamic friction of the polyester layer A ispreferably not more than 0.45 and can be adjusted so by the same methodas described above including addition of at least one member frompolymer particles, inorganic fine particles and thermoplastic resinincompatible with polyester to polyester. The coefficient of dynamicfriction is more preferably 0.40-0.20 and most preferably 0.35-0.20.When the coefficient of dynamic friction of the polyester layer A is notmore than 0.45, scratch resistance reaches a practical level and metalcontainers with good appearance can be obtained even if the canmanufacturing speed is increased, thus enhancing the productivity of canmanufacture.

The melting point of the polyester layer B is preferably 180°-240° C.and can be adjusted to fall within this range by suitably selecting thekind and amount of the copolymerizable components of the above-mentionedpolyester, with preference given to the use of a copolymer of ethyleneterephthalate and isophthalate. The melting point is more preferably200°-230° C. When the melting point of the polyester layer B is lowerthan 180° C., heat resistance becomes poor and wrinkles are developedduring laminating. When a laminated metal sheet is subjected to heattreatment such as retorting, the laminate film tends to turn white orpeel off. When the melting point exceeds 240° C., heat adhesivenessbecomes poor.

The thickness of the layer A of the aforementioned polyester film ispreferably 3-50 μm, more preferably 5-20 μm. When the thickness is lessthan 3 μm, handling of the film tends to become difficult, laminateprocessability becomes poor and flaws such as pin hole and crack tend tooccur during the can manufacture process. On the other hand, thethickness exceeding 50 μm tends to result in uneconomical, saturatedeffect of protection such as corrosion resistance of the metal sheet andadverse influence on adhesiveness due to a greater inner stress of thefilm itself. The thickness of the layer B of the aforementionedpolyester film is preferably 1-15 μm, more preferably 2-10 μm. When thethickness is less than 1 μm, adhesiveness with a metal sheet becomesinsufficient, while when it exceeds 15 μm, the adhesiveness with a metalsheet reaches saturation and heat resistance becomes poor.

The production of the polyester film composed of the abovementionedlayers A and B is subject to no particular limitation insofar as a filmsatisfying the requirements as described above is obtained. For example,such film is produced by multi-layer extrusion or extrusion laminate,with preference given to multi-layer extrusion from the economical pointof view.

The laminated metal sheet of the present invention is produced bylaminating the aforementioned polyester film on a metal sheet. Theusable metal sheet includes tin, tin-free steel or aluminum.

The above-mentioned polyester film may be laminated on the metal sheetby any method such as conventional dry laminating or thermal laminating.For example, an adhesive layer is formed on a polyester film and theadhesive side of the polyester film is laminated with a metal sheet. Theadhesive layer is preferably formed on the film in a partially set stateand allowed to completely set while being laminated on the metal sheet.The adhesive may be set by the use of heat, light or electron beam. Theadhesive to be used in the above method may be epoxy resin, polyurethaneresin, polyester resin, polyester polyurethane resin, isocyanate resinor modified resins of these resins.

When a multi-layer film composed of a polyester layer (layer A) having acoefficient of dynamic friction between the film and a metal at 80° C.of not more than 0.45 and a polyester layer (layer B) having a meltingpoint of 180°-240° C. is desired, the layer B is preferably fitted on ametal sheet and subjected to thermal laminating. In particular, athermal laminating by electrifying to heat the metal sheet ispreferable.

Said thermal laminating is the most preferable mode, since no organicsolvent remains so that adverse influences on the taste and smell of thefood by the residual solvent are preventable.

The polyester film may be laminated on either side or both sides of themetal sheet. When a both-side laminating is desired, a simultaneouslaminating or sequential laminating may be conducted.

The metal container of the present invention is obtained by forming theabove-mentioned laminated metal sheet. The method for forming the metalcontainer is not particularly limited. While the shape of the metalcontainer is not limited, a so-called three-piece can which ispreferable for packing retort food, coffee drinks or the like, whereinthe lids of head and ail are seamed to seal the content, is preferred.

The present invention is explained in more detail by referring toExamples, to which the present invention is not limited. The followingExamples are for illustrating purposes and any modification and changesmade within the scope of the present invention are encompassed in thetechnical range of the invention.

Various tests were conducted in Examples and Comparative Examples to bementioned later, according to the following methods.

(1) Coefficient of dynamic friction

A film sample was set on a skid weighing 1.5 kg and having a contactarea of 50 mm×70 mm and the coefficient of dynamic friction when theskid was glided on a tin-free steel sheet at 80° C. and 250 mm/min wasmeasured.

(2) Scratch resistance

Using a color fastness friction tester manufactured by Toyo Seiki,Japan, scratch resistance was measured. A friction runner set with afilm sample and applied with a load of 400 g was allowed to rub in thedistance of 100 mm at 30 reciprocations/min on a 80° C. tin-free steelsheet for one minute. The flaws on the film were visually observed andevaluated following the criteria below. The film evaluated with Δ or ∘is practical.

∘: Flaws were scarce.

Δ: Partial flaws were observed.

×: Flaws were observed on the entirety of the film.

(3) Content of cyclic ethylene terephthalate trimer

A polyester film was dissolved in hexafluoroisopropylalcohol/chloroform=2/3 (V/V) and polyester was deposited with methanolfor filtration. The filtrate was evaporated to dryness and the drysubstance was dissolved in dimethylformamide. This solution wasdeveloped by liquid chromatography and the cyclic ethylene terephthalatetrimer content in the polyester film was quantitatively determined. Inthe case of a multi-layer polyester film, the outermost layer oppositeto the side in contact with the metal surface of a laminated metal sheetto be obtained by the steps mentioned later was chipped off andsubjected to the determination.

(4) Elution of oligomer

A 10 cm-square laminated steel sheet was subjected to retort treatmentwith 500 cc of distilled water at 120° C. for 30 minutes. After thetreatment, the laminated steel sheet was air-dried and the surface ofthe laminated film on the steel sheet was examined with a magnifier.Elution of oligomer was evaluated following the criteria below.

observed: crystals of oligomer were found on the surface of the film

none: crystals of oligomer were not found on the surface of the film

(5) Melting point and glass transition temperature

A differential scanning calorimeter was used for the determination. Asample was melted by heating at 300° C. for 5 minutes and rapidly cooledwith liquid nitrogen. The temperature of 10 mg therefrom was raised at10° C./min. The specific heat change based on the transition from aglass state to a rubber state was read and the temperature of thetransition was taken as a glass transition temperature (Tg). Thetemperature of an endothermic peak based on melting of the crystals wastaken as a melting point.

(6) Average particle size

The determination was done by the Colter counter method.

EXAMPLE 1

A mixture of polyethylene terephthalate (97 parts by weight) having anintrinsic viscosity of 0.65 and comprising a cyclic ethyleneterephthalate trimer (0.38% by weight), which was obtained byheat-treating a mixture of polyethylene terephthalate chips (100 partsby weight) produced by melt polymerization, having an intrinsicviscosity of 0.65 and comprising a cyclic ethylene terephthalate trimer(1.0% by weight) and ethylene glycol (200 parts by weight) at 135° C.for 30 minutes; separating the polyethylene terephthalate chips; andsubjecting the chips to solid polymerization at 210° C. for 14 hoursunder the pressure of 0.1 mmHg, and a block copolymer (3 parts byweight) of polyethylene terephthalate-polytetramethylene glycol etherwas melt-extruded and sequentially subjected to biaxial orientation togive a 12 μm-thick polyester film. The cyclic ethylene terephthalatetrimer content of said polyester film was 0.40% by weight.

An adhesive (a mixture of ADCOTE, a polyurethane adhesive manufacturedby Toyo Ink, Japan and a curing agent) was applied on one side of thepolyester film at 4 g/m² (converted to solid) and dried. The film wassubjected to aging at 40° C. for 24 hours. The film was laminated on theboth sides of a degreased cold-drawn steel sheet by thermal laminatingto give a double laminated steel sheet.

The obtained double laminated steel sheet was placed in a pressurizercontaining distilled water and heated at 120° C. for 30 minutes toevaluate the model retort treatment. The laminated steel sheet treatedwas taken out from the pressurizer and the surface of the film wasobserved. The gloss of the film surface was fine and the oligomer didnot precipitate on the surface.

Comparative Example 1

By using a polyester film comprising a cyclic ethylene terephthalatetrimer (0.98% by weight) as obtained in the same manner as in Example 1except that polyethylene terephthalate comprising a cyclic ethyleneterephthalate trimer (1.0% by weight) and having an Intrinsic viscosityof 0.65, which was prepared by melt polymerization, was used, a film forlaminating was obtained as in Example 1. Using said film, a doublelaminated steel sheet was prepared as in Example 1 and subjected to theevaluation of model retort treatment in the same manner as in Example 1.The gloss of the film surface of the laminated steel sheet was poor andoligomer precipitated on the surface.

Example 2

A mixture of polyethylene terephthalate (90 parts by weight) comprisinga cyclic ethylene terephthalate trimer (0.33% by weight) as used inExample 1, and a copolymerized polyester (polyester A, 10 parts byweight, cyclic ethylene terephthalate trimer content 0.7% by weight)made from terephthalic acid/isophthalic acid (molar ratio: 70/30) andethylene glycol was used as the resin for layer A. As the resin forlayer B, used was the above-mentioned polyester A. The resin for thelayer A and the resin for the layer B were respectively melted in aseparate extruder and melt-extruded by the coextrusion method. Thesequential biaxial orientation gave a 20 μm-thick (the thickness oflayer A being 17 μm and that of layer B being 3 μm) biaxially-orientedcomposite polyester film. The cyclic ethylene terephthalate trimercontent of said layer A of the polyester film was 0.47by weight.

The layer B side of the polyester film was lapped over a tin sheet andpassed between metal rolls heated at 170° C. to give a laminated tinsheet.

The obtained laminated tin sheet was subjected to the evaluation ofmodel retort treatment In the same manner as in Example 1. The gloss ofthe film surface was fine and the oligomer did not precipitate on thesurface.

Comparative Example 2

In the same manner as in Example 1 except that a polyethyleneterephthalate comprising a cyclic polyethylene terephthalate trimer(1.0% by weight) was used, a biaxially-oriented composite polyester filmwas obtained. The cyclic polyethylene terephthalate trimer content ofsaid composite film was 0.90% by weight. The obtained composite film waslaminated with a tin sheet in the same manner as in Example 2 andsubjected to the evaluation of model retort treatment. The surface ofthe film on the laminated steel sheet treated as described showed poorgloss and oligomer precipitation on the film surface.

Example 3

Polyethylene terephthalate as a resin for layer A, a copolymer ofterephthalic acid/C36 dimer acid (molar ratio 85/15) and ethylene glycolas a resin for layer B and a copolymerized polyester of terephthalicacid/isophthalic acid (molar ratio 83/17) and ethylene glycol as a resinfor layer C were respectively melted in a separate extruder andmelt-extruded by the coextrusion method to give a 32 μm-thick (thethickness of layer A being 9 μm, that of layer B being 20 μm and that oflayer C being 3 μm) unoriented composite polyester film comprising acyclic ethylene terephthalate trimer (0.47% by weight).

The layer C side of the polyester film was lapped over a tin sheet andpassed between a metal roll heated at 230° C. and a rubber roll to givea laminated tin sheet.

The obtained laminated tin sheet was formed by a press under thepressure of 100 kg/cm² to give a 100 mm diameter, 70 mm deep cup. Thecup was subjected to the evaluation of model retort treatment in thesame manner as in Example 1. The gloss of the film surface of the cupwas fine and the oligomer did not precipitate on the surface.

Comparative Example 3

A cup made of a laminated steel sheet obtained in the same manner as inExample 3 by using the same unoriented composite polyester film exceptthat the cyclic polyethylene terephthalate trimer content in thecomposite film was 0.80% by weight, was subjected to the evaluation ofmodel retort treatment in the same manner as in Example 1. The surfaceof the cup treated as described showed poor gloss and oligomerprecipitation on the film surface.

Example 4

A mixture of polyethylene terephthalate (97 parts by weight) comprisingcohesive silica (0.1% by weight, average particle size 1.5 μm) andpolymethyl methacrylate particles (1.0% by weight, spherical, averageparticle size 3.0 μm) crosslinked with trimethylolpropanetrimethacrylate, which had an intrinsic viscosity of 0.65 and which wasmade to contain less amount of oligomer (cyclic ethylene terephthalatetrimer 0.33% by weight) by the same method as in Example 1, and a blockcopolymer (3 parts by weight) of polyethyleneterephthalate-polytetramethylene glycol ether was melt-extruded by aT-die method to give an amorphous sheet. The sheet was drawn 3.5-fold tothe longitudinal direction and 3.5-fold to the horizontal direction at90° C. and cured with heat at 200° C. to give a 12 μm-thick polyesterfilm.

An adhesive (a mixture of ADCOTE, a polyurethane adhesive manufacturedby Toyo Ink, Japan and a curing agent) was applied on one side of thepolyester film at 4 g/m² (converted to solid) and dried, The film wassubjected to aging at 40° C. for 24 hours. The film was laminated on theboth sides of a degreased cold-drawn steel sheet by thermal laminatingto give a double laminated steel sheet.

Example 5

In the same manner as in Example 4 except that polyethyleneterephthalate (97 parts by weight) comprising cohesive silica (0.3% byweight, average particle size 1.5 μm) and spherical zeolite (1.0% byweight) and showing almost monodisperse particle size distribution(average particle size 3.0 μm) was used in place of polyethyleneterephthalate (97 parts by weight) as used in Example 4, a polyesterfilm and a laminated steel sheet were obtained.

Example 6

In the same manner as in Example 4 except that a polystyrene resin (1.0%by weight) was used in place of the crosslinked polymethyl methacrylateparticles (1.0% by weight), a polyester film and a laminated steel sheetwere obtained.

Example 7

In the same manner as in Example 4 except that polyethyleneterephthalate (97 parts by weight) comprising spherical polystyreneparticles (1.0% by weight, average particle size 2.5 μm) crosslinkedwith divinylbenzene and showing almost monodisperse particle sizedistribution was used in place of polyethylene terephthalate (97 partsby weight) as used in Example 4, a polyester film and a laminated steelsheet were obtained.

Example 8

In the same manner as in Example 4 except that polyethyleneterephthalate (97 parts by weight) comprising cohesive silica (0.6% byweight, average particle size 1.5 μm) was used in place of polyethyleneterephthalate (97 parts by weight) as used in Example 4, a polyesterfilm and a laminated steel sheet were obtained.

Example 9

In the same manner as in Example 4 except that polyethyleneterephthalate (97 parts by weight) comprising cohesive silica (0.3% byweight, average particle size 1.5 μm) and low density polyethylene (1.0%by weight) was used in place of polyethylene terephthalate (97 parts byweight) as used in Example 4, a polyester film and a laminated steelsheet were obtained.

Example 10

In the same manner as in Example 4 except that polyethyleneterephthalate (97 parts by weight) comprising a cyclic ethyleneterephthalate trimer (0.4% by weight), cohesive silica (0.25% by weight,average particle size 2.4 μm) and butyl acrylate/methylmethacrylate/styrene particles (1.0% by weight) crosslinked withdivinylbenzene, the particles having an average particle size of 2.0μmand almost monodisperse particle size distribution, which had anintrinsic viscosity of 0.75 and which was prepared by solid phasepolymerization, was used in place of polyethylene terephthalate (97parts by weight) as used in Example 4, a polyester film and a laminatedsteel sheet were obtained.

Example 11

In the same manner as in Example 10 except that spherical silica (1.0%by weight, average particle size 2.0 μm) showing almost monodisperseparticle size distribution was used in place of butyl acrylate/methylmethacrylate/styrene particles (1.0% by weight) crosslinked withdivinylbenzene, a polyester film and a laminated steel sheet wereobtained.

Example 12

In the same manner as in Example 10 except that 6-nylon (1.5% by weight)was used in place of butyl acrylate/methyl methacrylate/styreneparticles (1.0% by weight) crosslinked with divinylbenzene, a polyesterfilm and a laminated steel sheet were obtained.

The properties of the polyester films and laminated steel sheetsobtained in the above Examples 4-12 are shown in Table 1.

The polyester films and laminated steel sheets obtained in Examples 4-12showed fine sliding property between the film and the metal at hightemperatures and superior scratch resistance. In addition, elution ofoligomer was not found, proving high quality as a film for laminatingwith a metal and as a laminated steel sheet.

The polyester films obtained in Examples 4-12 were applied to the insideof a can barrel, and the inside and outside faces of a bottom lid, and athree-piece can was manufactured. No scratch was found on the surface ofthe film during can production and high speed can production waspossible. Then, coffee was filled in said can and subjected to retorttreatment. There were found no migration of oligomer or organic solventfrom the film, no change in taste or smell of the coffee and noprecipitation of oligomer on the outer surface of the bottom lid.Accordingly, the quality of the can was high.

Comparative Example 4

In the same manner as in Example 4 except that crosslinked polymethylmethacrylate particles were not used, a polyester film and a laminatedsteel sheet were obtained. The properties of these are shown in Table 1.

The polyester film and a laminated steel sheet obtained were poor insliding property between the film and the metal at high temperatures andin scratch resistance. As a film for laminating with a metal and alaminated steel sheet, they had inferior quality. In the same manner asabove, a three-piece can was produced. During the can production, thesurface of the film developed scratch flaws and the product value waslow.

Comparative Example 5

In the same manner as in Example 4 except that polyethyleneterephthalate (97 parts by weight) comprising a cyclic ethyleneterephthalate trimer (1.0% by weight) and cohesive silica (0.1% byweight, average particle size 1.5 μm), which was produced by meltpolymerization, was used in place of polyethylene terephthalate (97parts by weight) as used in Example 4, a polyester film and a laminatedsteel sheet were obtained. The properties of these are shown in Table 1.

The polyester film and a laminated steel sheet obtained were poor insliding property between the film and the metal at high temperatures andin scratch resistance. In addition, elution of oligomer was found, thusproving low quality of the product. In the same manner as above, athree-piece can was produced and coffee was filled therein. During thecan production, the surface of the film developed scratch flaws andoligomer precipitation on the outer surface of the bottom lid upon theretort treatment. Accordingly, the product value was low.

Comparative Example 6

In the same manner as in Comparative Example 5 except that crosslinkedpolystyrene particles (0.% by weight) as used in Example 7 was used inplace of cohesive silica (0.1% by weight, average particle size 1.5 μm),a polyester film and a laminated steel sheet were obtained. Theproperties of these are shown in Table 1.

The polyester film and a laminated steel sheet obtained had a lowproduct quality as were those produced in Comparative Example 5. Athree-piece can was produced as mentioned above and coffee was filledtherein. The product quality was poor in Comparative Example 5,

                  TABLE 1                                                         ______________________________________                                                                  Property of                                         Properties of film        laminated                                           Coefficient             Cyclic ET steel sheet                                 of dynamic    Scratch   trimer (% Elution of                                  friction      resistance                                                                              by weight)                                                                              oligomer                                    ______________________________________                                        Ex. 4   0.28      ∘                                                                           0.38    none                                      Ex. 5   0.28      ∘                                                                           0.40    none                                      Ex. 6   0.31      ∘                                                                           0.41    none                                      Ex. 7   0.35      ∘                                                                           0.40    none                                      Ex. 8   0.39      Δ   0.40    none                                      Ex. 9   0.28      ∘                                                                           0.39    none                                      Ex. 10  0.28      ∘                                                                           0.51    none                                      Ex. 11  0.30      ∘                                                                           0.52    none                                      Ex. 12  0.32      ∘                                                                           0.52    none                                      Co. Ex. 4                                                                             0.65      x         0.40    none                                      Co. Ex. 5                                                                             0.65      x         1.0     observed                                  Co. Ex. 6                                                                             0.62      x         1.0     observed                                  ______________________________________                                         Note : ET = ethylene terephthalate                                       

Example 13

A mixture of polyethylene terephthalate (melting point 254° C., 97 partsby weight) comprising cohesive silica (0.1% by weight, average particlesize 1.5 μm) and polymethyl methacrylate spherical particles (1.0% byweight, average particle size 3.0 μm) crosslinked withtrimethylolpropane trimethacrylate, which had an intrinsic viscosity of0.70 and which was made to contain less amount of oligomer (cyclicethylene terephthalate trimer 0.33% by weight) by the method employed inExample 1, and a block copolymer (3 parts by weight) of polyethyleneterephthalate-polytetramethylene glycol ether was used as a resin forlayer A. As the resin for layer B, used was a copolymerized polyester(melting point 215° C.) of terephthalic acid/isophthalic acid (molarratio 83/17) and ethylene glycol, which comprised spherical silica (0.1%by weight) having an average particle size of 1.0 μm. The resin for thelayer A and the resin for the layer B were respectively melted in aseparate extruder and the molten resins were combined between the diesand extruded on a cooling drum to give an amorphous sheet. The sheet wasdrawn 3.5-fold to the longitudinal direction and 3.5-fold to thehorizontal direction at 90° C. and cured with heat at 200° C. to give a12 μm-thick polyester film (the thickness of layer A being 9μm and thatof layer B being 3 μm).

The layer B side of the polyester film was lapped over a degreasedcold-drawn steel sheet and passed between a metal roll heated at 230° C.and a rubber roll under the pressure of 20 kg/cm² to give a laminatedsteel sheet.

Example 14

In the same manner as in Example 13 except that polyethyleneterephthalate (97 parts by weight) comprising cohesive silica (0.3% byweight, average particle size 1.5 μm) and spherical zeolite (1.0% byweight, average particle size 3.0 μm) showing almost monodisperseparticle size distribution was used in place of polyethyleneterephthalate (97 parts by weight) as used for the resin for layer A inExample 13, a polyester film and a laminated steel sheet were obtained.

Example 15

In the same manner as in Example 13 except that polystyrene resin (1.0%by weight) was used in place of crosslinked polymethyl methacrylateparticles (1.0% by weight) as used for the resin for layer A, apolyester film and a laminated steel sheet were obtained.

Example 16

In the same manner as in Example 13 except that polyethyleneterephthalate (97 parts by weight) comprising spherical polystyreneparticles (1.0% by weight, average particle size 2.5 μm) crosslinkedwith divinylbenzene and showing almost monodisperse particle sizedistribution was used in place of polyethylene terephthalate (97 partsby weight) as used in Example 13 as the resin for layer A, and acopolymerized polyester (melting point 225° C.) of terephthalicacid/isophthalic acid (molar ratio 88/12) and ethylene glycol,comprising cohesive silica (0.1% by weight, average particle size 1.5μm), was used as the resin for layer B, a polyester film and a laminatedsteel sheet were obtained.

Example 17

In the same manner as in Example 13 except that polyethyleneterephthalate (97 parts by weight) comprising cohesive silica (0.6% byweight, average particle size 1.5 μm) was used in place of polyethyleneterephthalate (97 parts by weight) as used for the resin for layer A inExample 13, a polyester film and a laminated steel sheet were obtained.

Example 18

In the same manner as in Example 13 except that polyethyleneterephthalate (97 parts by weight) comprising cohesive silica (0.3% byweight, average particle size 1.5 μm) and low density polyethylene (1.0%by weight) was used in place of polyethylene terephthalate (97 parts byweight) as used for the resin for layer A in Example 13, a polyesterfilm and a laminated steel sheet were obtained.

Example 19

In the same manner as in Example 16 except that polyethyleneterephthalate (97 parts by weight) comprising a cyclic ethyleneterephthalate trimer (0.4% by weight), cohesive silica (0.25% by weight,average particle size 2.4 μm) and butyl acrylate/methylmethacrylate/styrene spherical particles (1.0% by weight) crosslinkedwith divinylbenzene, the particles having an average particle size of2.0 μm and almost monodisperse particle size distribution, which had anintrinsic viscosity of 0.75 and which was prepared by solid phasepolymerization, was used in place of polyethylene terephthalate (97parts by weight) as used for the resin for layer A in Example 16, apolyester film and a laminated steel sheet were obtained.

Example 20

In the same manner as in Example 19 except that spherical silica (1.0%by weight) having an average particle size of 2.0 μm and almostmonodisperse particle size distribution was used in place of butylacrylate/methyl methacrylate/styrene particles (1.0% by weight)crosslinked with divinylbenzene, a polyester film and a laminated steelsheet were obtained.

Example 21

In the same manner as in Example 19 except that 6-nylon (1.5% by weight)was used in place of butyl acrylate/methyl methacrylate/styreneparticles (1.0% by weight) crosslinked with divinylbenzene, a polyesterfilm and a laminated steel sheet were obtained.

The properties of the polyester films and laminated steel sheetsobtained in the above Examples 13-21 are shown in Table 2.

The polyester films and laminated steel sheets obtained in Examples13-21 showed fine sliding property between the film and the metal athigh temperatures and superior scratch resistance.

In addition, elution of oligomer was void, proving high quality as afilm for laminating with a metal and a laminated steel sheet.

The polyester films obtained in Examples 13-21 were applied to theinside of a can barrel, and the inside and outside faces of a bottom lidand a three-piece can was manufactured. No scratch was found on thesurface of the film during can production and high speed can productionwas possible. Then, coffee was filled in said can and subjected toretort treatment. There were found no migration of oligomer or organicsolvent from the film, no change in taste or smell of the coffee and noprecipitation of oligomer on the outer surface of the bottom lid.

Comparative Example 7

In the same manner as in Example 13 except that crosslinked polymethylmethacrylate particles were not used for the resin for layer A, apolyester composite film and a laminated steel sheet were obtained. Theproperties of these are shown in Table 2.

The polyester composite film and the laminated steel sheet obtained werepoor in sliding property between the film and a metal at hightemperatures and in scratch resistance. Accordingly, the product valueas a film for laminating with a metal and a laminated metal sheet waslow.

In the same manner as above, a three-piece can was produced. During thecan production, however, the surface of the film developed scratch flawsand the product value was low.

Comparative Example 8

In the same manner as in Example 13 except that polyethyleneterephthalate (97 parts by weight) having an intrinsic viscosity of 0.65and comprising a cyclic ethylene terephthalate trimer (1.0% by weight)and cohesive silica (0.1% by weight, average particle size 1.5 μm),which was produced by melt polymerization, was used in place ofpolyethylene terephthalate (97 parts by weight) as used in Example 13, apolyester composite film and a laminated steel sheet were obtained. Theproperties of these are shown in Table 2.

The polyester composite film and a laminated steel sheet obtained werepoor in sliding property between the film and the metal at hightemperatures and in scratch resistance. In addition, oligomer wascontained in a high proportion and large elution of the oligomer wasobserved, proving their low quality as a film for laminating with ametal and a laminated metal sheet.

In the same manner as above, a three-piece can was produced. During thecan production, the surface of the film developed scratch flaws. The canwas filled with coffee and subjected to a retort treatment. As a result,oligomer precipitated on the outer surface of the bottom lid.Accordingly, the product value was low.

Comparative Example 9

In the same manner as in Example 13 except that the resin for layer Awas used in place of the resin for layer B, a polyester composite filmwas obtained. Using the film obtained, a steel sheet was laminated inthe same manner as in Example 13. The adhesion strength was poor and thefilm obtained was not very practical.

Comparative Example 10

In the same manner as in Example 13 except that the resin for layer Bwas used in place of the resin for layer A, a polyester composite filmand a laminated steel sheet were obtained. The properties of these areshown in Table 2.

The polyester composite film and a laminated steel sheet obtained werepoor in sliding property between the film and the metal at hightemperatures and in scratch resistance. In addition, oligomer wascontained in a high proportion and large elution of the oligomer wasobserved, proving low quality as a film for laminating with a metal anda laminated metal sheet.

In the same manner as above, a three-piece can was produced. During thecan production, the surface of the film developed scratch flaws. The canwas filled with coffee and subjected to a retort treatment. As a result,oligomer precipitated on the outer surface of the bottom lid.Accordingly, the product value was low.

Comparative Example 11

In the same manner as in Example 13 except that a copolymerizedpolyester (melting point 170° C., determined by a melting temperaturemeasuring device manufactured by Metler) of terephthalicacid/isophthalic acid (molar ratio 67/33) and ethylene glycol,containing spherical silica (0.1% by weight) having an average particlesize of 1.0 μm was used as a resin for layer B, a polyester compositefilm was obtained. The film was laminated with a steel sheet in the samemanner as in Example 13. As a result, wrinkles were developed and thefilm was not practically used for laminating with a metal sheet.

Comparative Example 12

In the same manner as in Example 8 except that crosslinked polystyreneparticles (0.1% by weight) as used in Example 16 was used as the resinfor layer A in place of the cohesive silica having an average particlesize of 1.5 μm, a polyester composite film and a laminated steel sheetwere obtained. The properties of these are shown in Table 2.

The polyester composite film and a laminated steel sheet obtained hadpoor quality as were those obtained in Comparative Example 8. Athree-piece can was produced and coffee was filled therein. The canshowed low product value as in Comparative Example 8.

                  TABLE 2                                                         ______________________________________                                        Properties of film                                                            Layer A                                                                                             Cyclic        Property of                               Coeffi-               ET       layer                                                                              laminated                                 cient of     Scratch  trimer   B    steel sheet                               dynamic      resis-   (% by    MT   Elution of                                friction     tance    weight)  (°C.)                                                                       oligomer                                  ______________________________________                                        Ex. 13  0.28     ∘                                                                          0.38   215  none                                    Ex. 14  0.28     ∘                                                                          0.40   215  none                                    Ex. 15  0.31     ∘                                                                          0.41   215  none                                    Ex. 16  0.35     ∘                                                                          0.40   225  none                                    Ex. 17  0.39     Δ  0.40   215  none                                    Ex. 18  0.28     ∘                                                                          0.39   215  none                                    Ex. 19  0.28     ∘                                                                          0.51   225  none                                    Ex. 20  0.30     ∘                                                                          0.52   225  none                                    Ex. 21  0.32     ∘                                                                          0.52   225  none                                    Co. Ex. 7                                                                             0.65     x        0.40   215  none                                    Co. Ex. 8                                                                             0.65     x        1.0    215  observed                                Co. Ex. 9                                                                             0.66     x        0.38   254  none                                    Co. Ex. 10                                                                            0.68     x        0.78   215  observed                                Co. Ex. 11                                                                            0.65     x        0.38   170  none                                    Co. Ex. 12                                                                            0.62     x        1.0    215  observed                                ______________________________________                                         Note: ET = ethylene terephthalate                                        

As described in the foregoing, the polyester film of the presentinvention having a coefficient of dynamic friction between the film anda metal at 80° C. of not more than 0.45 is superior in sliding propertybetween the film and the metal at high temperatures. When cans aremanufactured from the film of the invention, the film shows superiorscratch resistance on the film surfaces during can production,permitting high speed can production. When the film comprising a cyclicethylene terephthalate trimer in a proportion of not more than 0.7% byweight is used, moreover, elution of oligomer from the film can besuppressed even upon heat treatment after packing food therein (retorttreatment), so that the change in taste and smell caused by themigration of the oligomer into the food or degraded appearance of thesurface of the film caused by the oligomer precipitation are avoided.

The polyester film of the present invention is suitably used forlaminating with a metal sheet and metal containers suitable for packingthe food can be manufactured from the laminated metal sheet.

What is claimed is:
 1. A polyester film adapted to be laminated to atin-free steel, said film having a coefficient of dynamic frictionbetween the film and tin-free steel at 80° C. of not more than 0.45. 2.The polyester film of claim 1, composed of a polyester comprising anethylene terephthalate unit in a proportion of not less than 70% bymole.
 3. The polyester film of claim 1, comprising a cyclic ethyleneterephthalate trimer, said trimer present in a proportion of not morethan 0.7% by weight per total amount of polyester.
 4. The polyester filmof claim 1, composed of a polyester composition comprising polymerparticles having an average particle size of 0.5-5 μm in a proportion of0.3-5% by weight, said polymer being at least one member selected fromthe group consisting of polymers of acrylic monomer, polymers of styrenemonomer, copolymers of acrylic monomer and styrene monomer, copolymersof styrene monomer and crosslinking monomer, copolymers of acrylicmonomer, styrene monomer and crosslinking monomer, melamine resins,benzoguanamine resins, phenol resins and silicone resins.
 5. Thepolyester film of claim 1, composed of a polyester compositioncomprising inorganic fine particles, having an average particle size of0.5-5 μm in a proportion of 0.3-5% by weight.
 6. The polyester film ofclaim 1, composed of a polyester composition comprising a thermoplasticresin incompatible with polyester in a proportion of 0.3-5% by weight.7. A polyester film adapted to be laminated to a tin-free steel, saidfilm comprising a first polyester layer having a coefficient of dynamicfriction between the film and tin-free steel at 80° C. of not more than0.45 and a second polyester layer having a melting point of 180°-240° C.8. A laminated metal sheet comprising the polyester film of any one ofclaims 1-7 laminated with said metal sheet.
 9. A metal container formedfrom the laminated metal sheet of claim
 8. 10. A polyester film adaptedto be laminated to aluminum, said film having a coefficient of dynamicfriction between the film and aluminum at 80° C. of not more than 0.45.11. The polyester film of claim 10, composed of a polyester comprisingan ethylene terephthalate unit in a proportion of not less than 70% bymole.
 12. The polyester film of claim 10, comprising a cyclic ethyleneterephthalate trimer, said trimer present in a proportion of not morethan 0.7% by weight per total amount of polyester.
 13. The polyesterfilm of claim 10, composed of a polyester composition comprising polymerparticles having an average particle size of 0.5-5 μm in a proportion of0.3-5% by weight, said polymer being at least one member selected fromthe group consisting of polymers of acrylic monomer, polymers of styrenemonomer, copolymers of acrylic monomer and styrene monomer, copolymersof acrylic monomer and crosslinking monomer, copolymers of styrenemonomer and crosslinking monomer, copolymers of acrylic monomer, styrenemonomer and crosslinking monomer, melamine resins, benzoguanamineresins, phenol resins and silicone resins.
 14. The polyester film ofclaim 10, composed of a polyester composition comprising inorganic fineparticles having an average particle size of 0.5-5 μm in a proportion of0.3-5% by weight.
 15. The polyester film of claim 10, composed of apolyester composition comprising a thermoplastic resin incompatible withpolyester in a proportion of 0.3-5% by weight.
 16. A polyester filmadapted to be laminated to aluminum, said film comprising a firstpolyester layer having a coefficient of dynamic friction between thefilm and aluminum at 80° C. of not more than 0.45 and a second polyesterlayer having a melting point of 180°-240° C.
 17. A laminated metal sheetcomprising the polyester film of any one of claims 10-16 laminated withsaid metal sheet.
 18. A metal container formed from the laminated metalsheet of claim 17.